Do you mean a backpack that accepts a 3l bladder? There's loads of vastly different sizes in that category, from running vests to ul hiking day-/backpacks. So, you might want to specify the size a little more?
I got an Aonijie C9112 for 35 bucks on some sale last year with coins/coupon (50ish without). It's a 12 l pack that will gladly take one of these cheap ali 3l fold-over bladders (Newboler and such) with quite a bit of space to spare for a softshell, bits and bobs, a bit of food or whatever. For some reason, they're now bumped up to 70ish USD on ali. I gladly paid what I paid for the pack (and would buy again), would consider it OK for up to 50ish USD, would absolutely not pay 70.
Judging from my highly scientific research (3,5 or so YT videos), Aonijie packs seem to be well regarded as budget options (except when there is a price hike going on).
